J . Am. Chem. SOC. 1986, 108, 1585-1588 (3 X 25 mL) and dried over anhydrous MgS04. Removal of solvent (rotary evaporator) gave 0.34 g of crude product. Distillation under reduced pressure afforded 0.18 g (36%) of a clear, colorless liquid: bp 68-69 OC (0.55 mmHg); PMR (CDC13) 6 2.01 (s, CH3 + y-H, 6 H), 1.86 (d, 0-H, 6 H, J = 2.4 Hz), 1.70 (d, 6-H, 6 H, J = 2.4 Hz); IR (neat) 2910, 2860, 1655, 1450, 1345, 1300, 1045 cm-’. Anal. Calcd for Cl,H18S:C, 72.46; H, 9.95. Found: C, 72.36; H, 10.06. Kinetic Procedures. Each run was carried out by using nine sealed Kimble Neutraglas ampules. For runs with HFIP-containing solvents, each ampule contained 1 mL of solution, and for all other runs, each ampule contained 5 mL of solution. For runs in water, alcohols, and aqueous-organic mixtures, ampules were removed from the constant temperature bath at suitable time interals, quenched in an ice bath, and the contents rinsed with acetone into 25 mL of acetone, containing Lacmoid (resorcinol blue) indicator, cooled within a solid C0,-acetone slush bath. The acid produced was titrated against a standardized solution of sodium methoxide in methanol. The titration procedures for runs in acetic acid and formic acid and the calculation of the first-order
1585
solvolytic rate coefficients were as previously described.2s Product Studies. Ampules containing a ca. 0.01 M solution of 1AdSMe,+OTf in ethanol or the appropriate aqueous-ethanol solvent were allowed to react for at least ten half-lives at 70.6 or 100.1 OC. The products were directly analyzed by response-calibrated GLC, as previously described.34Only I-adamantanol and I-adamantyl ethyl ether were detected as products; in particular, no I-adamantyl methyl sulfide was detected. The 1-adamantyl methyl sulfide was, however, detected after reaction in ethanol in the presence of a large excess of anhydrous60 tetraethylammonium chloride.
Acknowledgment. This research was supported, in part, by the award of a Doctoral Research Fellowship to S.W.A. by the N I U Graduate School. D.N.K. thanks T. W . Bentley (University College of Swansea, University of Wales) and H. M. R. Hoffmann (Universitat Hannover) for hospitality while this manuscript was being prepared.
Geometries and Energies of the Fluoroethylenes David A. Dixon,* Tadamichi Fukunaga, and Bruce E. Smart Contribution No. 3754, E . I. d u Pont de Nemours & Co., Central Research & Development Department, Experimental Station, Wilmington, Delaware 19898. Received May 3, 1985. Revised Manuscript Received October 2, I985
Abstract: The geometries of the fluorinated ethylenes have been gradient optimized at the S C F level with a double-< plus polarization function on carbon (DZ+Dc) basis set. The C==C and C-F bond lengths for C2H4,C,H,F, CH,CF2, cis-CHFCHF, trans-CHFCHF, and C2F4have been optimized at the configuration-interaction level, including all single and double excitations (CI-SD). Good agreement with the original electron diffraction work was found. The values for r(C=C) and r(C-F) are found to decrease with increasing fluorine substitution. Correction factors to the S C F values are discussed and an estimated structure for C2HF3is given. Isodesmic reaction ethalpies for the fluoroethylenes have been calculated with (DZ+D,-) and double-< plus polarization function (DZ+P) basis sets. Both basis sets gave comparable results in satisfactory agreement with experiment. The calculated heats of formation of cis- and trans-CHFCHF are -71.0 and -70.0 kcal/mol, respectively, which compare to the experimental values of -70.8 f 3.1 and -69.4 f 3.1 kcal/mol. Ionization potentials and dipole moments of the fluoroethylenes also have been calculated and are compared to experimental data.
Fluoroethylenes are very simple compounds and are extremely important monomers, but their structures are not unequivocally established.’ In a systematic electron diffraction study, Bauer and co-workers2 found that the C=C bond length decreases as fluorines are substituted for hydrogen in ethylene. The C==C bond length of 1.315 A in CH2CF2from a microwave study3a agrees with an electron diffraction value of 1.316 A.* The bond distances in C2H,F as determined by microwave3b and electron diffraction were also in good agreement. Recently, however, the structures of the fluoroethylenes have been redetermined by using a combination of electron diffraction and microwave and the results did not show the expected general trend for a decrease in (1) Smart, B. E. In ‘Molecular Structures and Energetics”;Liebman, J. F., Greenberg,A., Eds.; Verlag Chemie: Deerfield, FL, 1986; Vol. 3, Chapter 4.
(2) Carlos, J. L.; Karl, R. R.; Bauer, S. H. J . Chem. Sor., Faraday Trans 2, 1974, 70, 177. (3) (a) Laurie, V. W.; Pence, D. T. J . Chem. Phys. 1963, 38, 2693. (b) Laurie, V. W. J . Chem. Phys. 1961, 34, 291. (4) Huisman, P. A. G.; Mijlhoff, F. C.; Renes, G. H. J . Mol. Struct. 1979, 5 1 . 191. 1~
(5) Spelbos, A.; Huisman, P. A. G.; Mijlhoff, F. C.; Renes, G. H. J . Mol. Struct. 1978, 44, 159. (6) VanSchaick, E. J. M.; Mijlhoff, F. C.; Renes, G. H.; Geise, H. J. J . Mol Struct. 1974, 21, 17. (7) Mijlhoff, F. C.; Renes, G. H.; Kohata, K.; Oyanagi, K.; Kuchitsu, K. J . Mol. Struct. 1977, 39, 241. ( 8 ) Mom, V.; Huisman. P. A. G.; Miilhoff, F. C.; Renes. G. H. J . Mol. Struct. 1980, 62, 9 5 .
0002-7863/86/ 1508-1 585$01.50/0
the value of r(C=C) with increasing substitution of fluorine. In fact, the C=C bond lengths in CH2CH2,CH2CF2,and CHFCF, were reported to be identical within experimental error.’,* The similarity in values for r(C=C) and r(C-F) further complicated the analysis of the electron diffraction data. As part of our general theoretical study of fluorocarbons, we have optimized the structures of the fluoroethylenes a t the SCF level using a double-{ (DZ) basis set augmented by polarization functions on carbon (DZ+D,-). T o obtain accurate geometries and resolve the discrepancies among the experimental measurements, we subsequently optimized the C=C and C-F bonds for C2H4, C2H3F,the difluoroethylenes, and C2F4with correlated wave functions starting from the optimum SCF structures. The results are compared with those from previous a b initio calculations. To further test the reliability of the DZ+Dc basis set, the total energies of the fluoroethylenes were computed and used to calculate isodesmic reaction enthalpies, which are known experimentally. Ionization potentials and dipole moments also were calculated and compared to experiment. Calculations. The calculations were performed with the H O N D O program’ package on D E C VAX/ 11-780 and IBM 3083 computers. Geometries were optimized a t the SCF level with the use of gradient techniques.I0 The correlated wave (9) (a) Dupuis, M.; Rys, J.; King, H. F. J . Chem. Phys. 1976,65, 11 1. (b) King, H. F.; Dupuis, M.; Rys, J. National Resource for Computer Chemistry Software Catalog; Program Q H 0 2 (HONDO), 1980; Vol. 1.
0 1986 American Chemical Society
1586 J . Am. Chem. SOC.,Vol. 108, No. 7 , 1986 functions were obtained at the level of a configuration interaction calculation including all single and double excitations (CI-SD) from the valence space to the virtual space. The C and F Is core electrons were frozen in the calculation. The orbitals for the CI were obtained from a single-determinant Hartree-Fock calculation. The C=C and C-F bonds were optimized parabolically, starting from the S C F optimum geometries. The basis set for these calculations is of double-{ quality in the valence space with exponents and coefficients from Dunning and Hay." The basis set is augmented by a set of d polarization functions on each carbon" and has the form (9,5,1/9,5/4)/[3,2,1/3,2/2] in the order C, F, H. This basis set gives good structures a t the S C F level and previous workI2 has shown that d orbitals on C are significantly more important than d orbitals on F at the S C F level. This basis set is also of tractable size for the C I calculations.
Results and Discussion Geometries. The S C F parameters for the fluoroethylenes are given in Table I and are compared with the experimental values. The bond angles and C-H bond distances in general agree well with experiment. The only major discrepancies (excluding C2HF3) are the value of O(H2C,C2)in C2H3F,2B(H,C2C,) in C2H3F,3b,4 and B(CCH) in trans-CHFCHF.2 These differences are all in the angles involving hydrogen which are not precisely determined by experiment. For C2HF3,the agreement with the angles determined by Mijlhoff and co-workerss is good; however, the agreement with the angles given by Bauer and associates2 is not as good. This is probably because of errors in their structure since they assumed that all of the C-F bond lengths were equivalent. The C = C and C-F bond lengths at the S C F level are shorter than the experimental values, as expected. The S C F values for r(C=C) generally decrease with increasing fluorine substitution. This is especially pronounced for gem-difluoro substitution. The C-F bond lengths also decrease with increasing fluorine substitution and the effect is again most pronounced for gem-difluoro substitution. The S C F values are in agreement with the trend in r(C=C) determined by the Bauer group.* There have been several theoretical calculations on the structures of the fluoroethylenes. Bock and ~c-workers'~ made the most extensive comparison and determined the structures of the ethylenes with 0, I , and 2 fluorines using a small double-l basis set. The C=C bond lengths a t this level are too short, even in comparison with the structures determined with the DZ+Dc basis set, and the values for r(C-F) are all too long, even when compared to the experimental values. Optimized geometries determined with both the 4-31Gi4and 3-21GI5 basis sets similarly exhibited overly contracted C=C bonds and elongated C-F bonds. The trend of decreasing r(C=C) with increasing fluorine substitution, however, was found with each of these basis set^.'^-'^ Dykstra and co-workersI6 optimized the structures of cis- and trans-CHFCHF with a fully polarized double-{ basis set (DZ+P). Excellent agreement with our DZ+Dc calculations is found, except for the C-F bond length which is shorter a t the DZ+P level by 0.008-0.009 A. This again is the expected result of increasing the size of basis set and also has been observed in comparisons of DZ+Dc and DZ+P calculations on the fluoromethanes.12 The values for r(C=C) and r(C-F) determined at the level of a CI-SD calculation are also given in Table I. When the CI correction is included, the bonds lengthen, as would be expected from previous studies. The C% bond lengths increase uniformly (10) Pulay, P. In 'Applications of Electronic Structure Theory"; Schaefer, H. F., 111, Ed.; Plenum Press: New York, 1977; Chapter 4. (1 1) Dunning, T. H., Jr.: Hay, P. J. In "Methods of Electronic Structure Theory"; Schaefer, H. F., 111, Ed.; Plenum Press: New York, 1977; Chapter 1. (12) Dixon, D. A,, unpublished results. (13) Bock, C. W.; George, P.; Mains, G. J.; Trachtman, M. J . Cfiem.SOC., Perkin Trans 2 1979, 814. (14) Frenking, G.; Koch, W.; Schaale, M. J . Compur. Chem. 1985,6. 189. (15) Whiteside, R . A.; Frisch, M. J.; DeFrees, D. J.; Schlegel, H . 9.; Raghavachari, K.; Pople, J . A. "Carnegie-Mellon Quantum Chemistry Archive", 2nd ed.; Carnegie-Mellon University: Pittsburgh, I98 1. (16) Gandhi, S. R.; Benzel, M . A.; Dykstra, C. E.; Fukunaga, T. J . Phys. Cfiem. 1982, 86, 3121.
Dixon et al. Table 1. Geometric Parameters for Flourinated Ethylenes" parameter
expt l b
expt 2
expt 3
C2H4 1.339< 1.086 117.6
r(C=C) r(C-H) B(HCH)
SCF
CI-SD
1.325 1.076 116.8
1.342
1.330 1.359
r(C=C) r(C-F)
1.333 1.348
1.330d 1.351
1.332c 1.348
1.314 1.338
r(C-H,) r(C-H2) r(C-H,) B(H,C~C~) B(H2CiG) ~(H3C2Cl) WCIC2)
1.090 1.085 1.076 121.4 123.9 127.7 121.0
1.108 1.097 1.107 120.4 118.7 130.8 121.5
1.086 1.079 1.071 120.7 118.8 120.9 121.0
1.074 1.073 1.073 121.6 119.3 126.0 122.6
r(C=C) r(C-F)
1.316 1.324
CH2=CF2 1.340g 1.315h 1.315 1.323
1.307 1.310
r(C-H) B(FCF) B(HCH)
1.075 109.7 119.3
1.091 110.6 122.0
1.072 109.2 120.6
r(C=C) r(C-F)
1.331 1.335
r(C-H) B(CCF) B(CCH)
1.084 123.7 121.6
r(C=C) r(C-F)
1.329 1.344
r(C-H) B(CCF) B(CCH)
1.080 119.3 129.3
1.088 119.8 125.0
1.072 120.1 125.5
r(C=C) r(C-F,) r(C-F2) r(C-F,) r(C-H) B(FIClC2) B(F2CIC2) ~(F3C2Cl) WC*C,)
1.309 1.336 1.336 1.336 1.073 125.4 125.4 118.8 127.2
1.341' 1.316 1.316 1.342 1.100 124.0 123.1 120.0 124.0
1.307 1.304 1.310 1.331 1.070 126.0 122.8 121.2 123.1
r(C=C) r(C-F) NFCF)
1.311 1.319 112.5
(1.51)'
1.079 109.1 121.9
cis-CH F=C H F 1.330' 1.324h 1.342 1.335
1.312 1.331
1.323 1.330 ( I ,322)'
1.329 1.349 (1.341)(
1.103 122.0 124.1
1.089 122.1 124.0
rrans-CH F=CH F 1.32W 1.338
1.071 122.6 123.1 1.311 1.336
1.307 1.306 112.6
1.328 1.354 (1.346)'
(1.323)' (1.315)' (1.321)' (1.342)'
1.320
1.322 ~~
~
~~
"Bond distances in A, bond angles in degrees. *Reference 2. CHarmony, M. D.; Laurie, V. W.; Kuczkowski, R. L.; Schwendeman, R . H.; Ramsay, D. A.; Lovas, F. J.; Lafferty, W. J.; Maki, A. G . J . Phys. Cfiem. Ref. Dura 1979, 8, 619. dReference 4. 'Reference 3b. 'Corrected values obtained by subtracting 0.008 A from the CI-SD value. See text. gReference 5. hReference 3a. 'Reference 6. 'Reference 7. 'Reference 8. 'Estimated from the S C F values with use of correction values of +0.016 8, for r(C=C) and +0.011 A for r(C-F). See text.
by -0.016 A, whereas the C F bond lengths show a slightly larger increase of -0.019 A.I7 The smallest increases for both values are found for C2F,.'* The correlation correction gives exactly (17) Comparison with experiment is complicated by the variety of distance parameters reported. We calculate re and the various reported values may vary by up to 0.01 A from this value. See: Yokozeki, A , ; Bauer, S. H. Top. Curr. Chem. 1975, 53, 72.
J . Am. Chem. SOC.,Vol. 108, No. 7 , 1986
Geometries and Energies of the Fluoroethylenes
Table 11. Total Energies (au) of the Fluoroethylenes Determined at the Optimum S C F Geometry (DZ+Dc) molecule SCF(DZ+D,) S C F( DZ+P) CI-SD(DZ+Dc) -78.042712 -78.050701 -78.295 362 C2H4 -176.912 823 -176.929856 -177.251 795 CZH3F CH2CFl -275.785037 -275.814 166 -276.207 838 cis-CHFCHF -275.771 009 -275.798 143 -276.193 007 trans-CH FCH F -275.771 357 -275.797 942 -276.193450 -374.635 360 -374.674 803 C2HF3 -473.492467 -473.544715 -474.074 774 CF,
Table 111. Isodesmic Reaction Energies AHo/AE (kcal/mol) ea
3-21G"
DZ+D,
DZ+P
AHo(exutl)*
1 2 3 4 5 6
5.014.6 -5.01-6.4 7.716.9 -17.71-17.9 -22.61-24.2 -12.7/-13.3
1.7/ 1.3 -6.71-8.1 5.214.4 -13.61-13.8 -20.31-21.9 -11.8/-12.4
3.613.2 -4.41-5.8 6.415.8 -14.71-14.9 -19.11-20.7 -11.0/-11.6
1 . 1 f 1.9 -5.0 f 3.1 3.6 f 5.2 -9.8 f 4.0 -14.6 f 2.3 -8.6 f 3.5
"Total energies determined at the optimum 3-21G geometry. The energies for CzH4,C2H,F, and CH,CF, are from ref 15. The energies for C2HF, and CzF4are -372.545 89 and -470.855 34 au, respectively (this work). bCalculated from experimental AHt' values in Table IV.
the same trends that are observed a t the S C F level: the va[ues for r(C=C) and r(C-F) decrease with increasing fluorine substitution. The C=C bond lengths at the CI level are in excellent agreement with those determined by the Bauer group.2 The C-F bond lengths are all somewhat longer than the experimental values. This is probably because of errors a t the S C F level where the C-F bonds are 0.008 A too long as compared to the DZ+P calculated values. When this correction is applied to the CI values (except for C2F4),the C-F bond lengths are now in excellent agreement with experiment (Table I). We also have estimated r(C=C) and r(C-F,) for C2HF3using a correction of +0.016 A for r(C=C) and a correction of +0.011 8, for r(C-F) (0.019-0.008 A) to give the values in Table I. The r(C=C) and r(C-F) values for each fluoroethylene that we consider to be most accurate are italicized. Energetics. The total S C F energies of the fluoroethylenes were determined a t the S C F (DZ+Dc) optimized geometries with the DZ+Dc basis set and with a basis set that included polarization functions on all atoms (DZ+P)." The values, which are listed in Table 11, were used to calculate the energies of the isodesmic reactions in eq 1-6. The results are given in Table 111. (The S C F isodesmic AE values calculated with the DZ+P basis set with use of the most accurate geometries from Table I rather than the DZ+Dc optimized geometries gave very similar results: 2.6, -7.0, 5.2, -14.8, -21.8, -12.2 kcal/mol for eq 1-6, respectively.) C2H4 + CH2CF2 2C2H3F (1)
-
-+
+ C2F4 C2H3F + C2HF3 2C2HF3 CH2CF2 + C2F4 C2HF3 + C2HSF 2CH2CF2
C2H4
-
-+
C2H4 C2H4
+ C2F4
+ C2HF3
-+
2CHZCF2
CHiCF2
+ C2H3F
(2) (3) (4) (5)
(6)
To properly compare the theoretical results to experiment, the calculated A E values need t o be converted to AH values. T h e required zero-point energies for the fluoroethylenes were calculated with the 3-21G basis set'9 and scaled values are given in Table IV. The calculated isodesmic reaction energies corrected for zero-point energy differences are compared with experiment and 3-21G results in Table 111. ( 1 8) The largest errors would be expected for C2F4because the basis set size is the smallest considering the number of electrons. The CI at the SD level also recovers the smallest percentage of correlation energy for C2F4,again due to the large number of electrons. (19) Binkley, J . S . ; Pople. J . A,; Hehre, W. J. J . A m . Chem. SOC.1980, 102, 939.
1587
Cl-SDQ(DZ+D,) -78.318555 -177.286675 -276.255 453 -276.240 925 -276.241 486 -474.151 803
The calculated AHo values for reactions 1-4 and 6 with both of our basis sets agree well with experiment. The 3-21G results are less satisfactory. The DZ+P results for reactions 1 and 4 are slightly outside the experimental errors. The greatest discrepancy between theory and experiment appears in reaction 5. The error is 4.5 kcal/mol with the DZ+P basis set and 5.7 kcal/mol with the DZ+Dc basis set. Among the five isodesmic equations, the neglect of correlation energy is also likely to produce the largest error for eq 5 . Because of the large number of fluorine valence electrons, CI-SD calculations do not recover as much of the correlation energy as that found for systems with fewer valence electrons. This occurs because higher order excitations play a larger role, simply because of the very large number of such excitations. Furthermore, size consistency problems can become significant. This will be especially true for isodesmic reactions involving C2F4. We therefore do not recommend applying a C I correction involving only single and double excitations to the isodesmic energies. (Such problems, however, are not expected to play a major role in the calculations of geometries.) For example, using the CI energies given i n Table 111, we calculate AHo = -27.0 kcal/mol at the CI-SD level and -23.8 kcal/mol at the CI-SDQ level for isodesmic reaction 5 . The latter calculation includes an estimate for quadrupole excitations using Davidson's formula.20 Thus, inclusion of higher order excitations gives a result in the right direction, but a large error is still present. The energy difference between cis- and trans- 1,2-difluoroethylene has been determined quite precisely from experiment.2' Contrary to simple chemical intuition, the cis isomer is more stable than the trans isomer by 0.93 f 0.03 kcal/mol (AAHfo), There have been numerous theoretical studies of this energy difference,13*14,16,22 with the most extensive one being that of Dykstra and co-workers.16 At the DZ+Dc level (Table 11) the trans structure is more stable by 0.22 kcal/mol. There is essentially no correlation correction with this basis set, AE(CI-SD) = 0.28 kcal/niol and LE(C1-SDQ) = 0.35 kcal/mol (Table 11). As expected from previous results,I6 improvement of the basis set leads to a better LIE. At the DZ+P level the cis isomer is correctly calculated to be more stable than the trans, but by only 0.13 kcal/mol. The experimental heats of formation of cis- and trans- 1,2difluoroethylene are not known precisely (Table IV). The trans isomer is predicted experimentally to be 10.7 f 3.9 kcal/mol less stable than CH2CF2. With the DZ+Dc basis set, the energy difference is 8.6 kcal/mol, favoring CH2CF2. Again there is essentially no correlation correction for this energy difference, LE(Cl-SD) = 9.0 kcal/mol and AE(C1-SDQ) = 8.8 kcal/mol. Improvement of the basis set leads to a small change in A/?,which is 10.1 kcal/mol with the D Z + P basis set. After correcting for Lero-point energy differences, our best estimates of the heats of formation are -70.0 kcal/mol for trans-CHFCHF and -7 1 .O kcal/mol for cis-CHFCHF. (20) Langhoff, S. R.; Davidson, E. R. I n ! . J . Quantum Chem. 1974, 8, 61. (21) (a) Entemann, E. A.; Craig, N. C. J . Am. Chem. Soc. 1961,83, 3047. (b) Craig, N . C.; Overend, J. J . Chem. Phys. 1969, 51, 1127. (c) Craig, ii, C.; Piper, L. G.; Wheeler, V . G. J . Phys. Chem. 1971, 75, 1453. (22) (a) Cremer, D. J . Am. Chem. SOC.1981, 103, 3633. (b) Cremer, D. Chem. Phys. Lett. 1981, 81, 481. (c) Skancke. A,: Boggs. J. E. J Am. Chem. SOC.1979, 101, 4063. (d) Binkley, J . S.: Pople, J. A . Chem. Phys. Lerr. 1977. 45, 197. (e) Whangbo, M.-H.; Mitchell, D. J.; Wolfe. S. J . Am. Chem. SOC. 1978, 100, 3698. (f) Bernardi, F.; Bottani, A,; Epiotis, N.D.; Guerra, M .J . Am. Client. Soc. 1976, 100, 6018. (g) Moffat, J. B. T H E O C H E M 1982. 5, 325.
1588
Dixon et al.
J . Am. Chem. SOC.,Vol. 108, No. 7, 1986
Table IV. Heats of Formation, Zero-Point Energies, and Dipole Moments li,‘
ethylene CZH, C2H3F
CH,CF, trans-CHFCHF cis-CHFCHF C2HF3 C2F4
AHf’
ZPE’
12.5 f 0.3 -33.2 f 0.4 -80.1 f 0.8 -69.4 f 3.1‘ -70.8 f 3.1‘ -1 17.2 f 2 -157.9 f 0.4
30.9 (30.9) 27.1 22.9 (22.1) 22.8 (22.5)e 23.1 (22.7) 18.5 13.3 (13.3)
D
(DZ+Dc)
(DZ+P)
(CI-SD)
exptl
1.92 1.82
1.67 1.44
1.73 1.62
1.43 1.38
3.26 I .79
2.87 1.s2
2.94
2.42 1.40
In kcal/mol. Unless noted otherwise, from: Pedley, J. B.; Rylance, J . “Sussex-X.P.L. Computer Analyzed Thermochemical Data: Organic & Organometallic Compounds”; University of Sussex: Sussex, Brighton, 1977. 3-21G calculated values multiplied by 0.90, in kcal/mol. Experimental values in parentheses were calculated from vibrational frequency data: Shimanouchi, T. ”Tables of Molecular Vibrational Frequencies”, NSRDSNBS 39: US. Government Printing Office: Washington, DC, 1972. ‘Calculated dipole moments at the SCF minimum geometry (DZ+D,). Experimental values from: Nelson, R. D., Jr.; Lide, D. R., Jr.; Maryott, A. A. “Selected Values of Electric Dipole Moments for Molecules in the Gas phase”; NSRDS-NBI 0: U.S. Government Printing Office: Washington, DC, 1967. McClellan, A. L. “Tables of Experimental Dipole Moments“; W. H. Freeman: San Francisco, 1963; Vol. 1. ‘Jochims, H. W.; Lohr, W.;Baumgartel, H. i2’ouc. J . Chim. 1979, 3, 109. ‘Reference 21b.
’
a
The effects of fluorination of t h e geometries a n d stabilities of ethylenes and the diverse explanations for these effects have been reviewed elsewhere.’ Among the various proposals to account for the relative stabilities of the three C2H2F2isomers are nonbonded a t t r a ~ t i o n , ~conjugative ~ ~ . ~ . ~ ~ d e ~ t a b i l i z a t i o n ,resonance ~~ stabiliz a t i ~ n , ~hyperconjugation,26 ’~.~~ as well as e l e c t r ~ s t a t i c l ~and .’~ other electronic effects.*’ It is not our intention t o try to sort out and pinpoint the specific electronic effects that determine fluoroethylene geometries and energies. Rather, we seek to identify the requirements for a basis set that a r e crucial for reasonably accurate calculations of geometries and energies. It is noteworthy, however, the whatever the actual electronic reasons might be, the relative stabilities of t h e fluoroethylenes appear to be entirely a consequence of their relative r-bond strengths. From t h e Dro values of 59.1, 62.8, a n d 5 2 . 3 kcal/mol for CH2CH2,C H 2 C F 2 , and CF2CF2,28 respectively, DTo(CF2=CF2)
+ DT0(CH2=CH2) - 2Dro(CH2=CF2)
=
-14.2 kcal/mol Within experimental error, this result is identical with the enthalpy
of isodesmic reaction 5. Similarly, D,0(CHZ=CF2) - D , O ( C - C H F = C H F ) ~ ~= 8.4 kcal/mol
This difference compares to t h e above DZ+Dc computed value of 8.8 kcal/mol for t h e difference in energy (AE) between cis-
CHFCHF and CH2CF2. Ionization Potentials and Dipole Moments. T h e ionization potentials of the fluoroethylenes are all known experimentally and (23) (a) Epiotis, N. D. “Lecture Notes in Chemistry, No. 29. Unified Valence Bond Theory of Electronic Structure”; Springer-Verlag: New York, 1982. (b) Epiotis, N. D.; Cherry, W. R.; Shaik, S.; Yates, R. L.; Bernardi, F. Top. Citrr. Chem. 1977, 70, 1. (c) Epiotis, N. D.; Yates, R. L. J . Am. Chem. SOC.1976, 98, 461. (24) Bingham, R. C. J . Am. Chem. SOC.1976, 98, 535. (25) (a) Viehe, H. G. Chem. Ber. 1963, 93, 953. (b) Pitzer, K. S.; Hollenberg, J . L. J . Am. Chem. SOC.1954, 76, 1493. (26) (a) Schleyer, P. v. R.; Kos, A. J. Tetrahedron 1983, 39, 1141. (b) Hoffman, R.; Radom, L.; Pople, J. A,; Hehre, W. J.; Salem, L. J . Am. Chem. Soc. 1972, 94, 6221. (27) (a) Snyder, W. H.; Hollein, H. C. J . Mol. Srrucr. 1982, 84, 83. (b) Kollman. P. A. J . Am. Chem. SOC.1974, 96, 4363. (28) (a) Wu, E.-C.; Rodgers. A. S. J . Am. Chem. Soc. 1976,98,6112. (b) Pickard, J. M.; Rodgers, A . S. J . Am. Chem. Sor. 1977, 99, 695. (29) D,’(c-CHF=CHF) is not known experimentally. The value for D,0(CH2=CH2) - D,O(c-CHF=CHF) was equated to the difference in activation energies for the cis-trans isomerizations of CHD=CHD and CHF=CHF (4.7 kcal/m01)~~ (the difference in r-bond energies between CF2=CH2 and CH2=CH2, 3.7 kcal/mol, is equivalent within experimental error to the computed difference in their rotational barriers, 3 kcal/mo13’). Thus, D,O(c-CHF=CHF) 2 54.4 kcal/mol. (30) Jeffers, P. M . J . Phys. Chem. 1974, 78, 1469. (31) Nagase, S.; Morokuma, K. J . Am. Chem. Soc. 1978, 100, 1661.
Table V. Ionization Potentials in eV DZ+D, ethylene
DZ+P
exptl”
10.26 10.51’ 10.27 10.56 10.58 10.48 C2H3F CHZCF2 10.95 10.74 10.69 cis-CHFCHF 10.84 10.64 10.44 trans-CH FCH F 10.84 10.64 10.38 10.54 11.17 10.85 C2HF3 11.05 10.56 11.48 C2F4 ”Reference 32. ’Levin, R. D.; Lias, S. G. “Ionization Potential and Appearance Potential Measurements 1971-1981”; NSRDS-NBS 71: U S . Government Printing Office: Washington, DC, 1982. See this reference for other values for the fluoroethylenes. C2H4
a r e about 10.5 eV.32 The calculated IP’s from Koopmann’s theorem are given in Table V and show excellent agreement with experiment considering the approximations involved. As expected, t h e DZ+P values show slightly better agreement. Although the fluoroethylene IP‘s are about equal, one trend is evident. For the C2H2F2isomers, t h e highest IP corresponds to t h e most stable isomer (CH2CFZ)and the lowest IP corresponds to the least stable isomer ( t r a n s - C H F C H F ) . T h e differences in the I P S are similar to the differences in the AHfo’s for the isomers. The I P ( C H 2 C F 2 ) is 7.1 kcal/mol higher t h a n I P ( t r a n s - C H F C H F ) , which approaches t h e difference of I O kcal/mol in AHfo’s. T h e IP difference between the cis and trans isomers is 1.4 kcal/mol, which compares to t h e difference in AHfo’s of 0.9 kcal/mol.2’ T h e experimental and calculated dipole moments, which were determined a t the minimum SCF geometry (DZ+Dc), are given in Table IV. The DZ+Dc values a r e roughly 30% high, whereas t h e DZ+P values a r e about 15% high. A s expected, including correlation lowers t h e dipole moment, but t h e effect is slightly smaller t h a n that found by improving the basis set.
Conclusions This study demonstrates that t h e geometries of the fluoroethylenes exhibit simple trends. T h e DZ+D, basis set gives a good description of the structure of the fluoroethylenes with errors like those typically found in other SCF calculations with good basis sets. Correlation corrections a t the CI-SD level provide a uniform correction and give very good structural parameters in comparison to experiment. We expect that our structures a r e accurate to within * l o for bond angles a n d fO.O1 A for bond lengths, which a r e typical experimental errors. Finally, we have demonstrated the adequacy of the D Z + D c basis set for predicting reasonable, consistent geometries a n d energies for these fluorocarbons. This basis set is still of a modest enough size that it can be applied to larger fluorocarbon systems. (32) Sell. J. A.; Kupperman, A. J . Chem. P h i s . 1979, 71, 4703
